I'm no expert in the arts and mysteries of firearms, but if I had a .54 I'd be concentrating on four-legged targets rather than those of the winged variety. I believe that a higher gauged smoothbore would serve you better in the pursuit of clay birds.

Smoke,
I would use 50 grains of FFG, a card wad, a cushion wad, an oz. of whatever shot equal to the powder charge, and an over wad. Use FFFFg powder in the pan and only prime to just below the touch hole. Tend to your flint and touch hole and swab your bore out every few shots. If you like shooting on the wing and think of hunting I would recommend that you read Chris Gilgun's article in Buckskinner #8 on page 100, "Wing Shooting with a Flintlock". Try follow thru when shooting at the bird, you'll come from behind keep your gun in motion and shoot as you see the bird but pull ahead after you fire. Remember that flintlocks have a faster ignition then percussion. I know that this is just a fun try out for you so those are the basics.
I hunt with a 12 bore club butt fowler and use # 6 shot for grouse. I also have a double flinter that I use # 8s for Woodcock. My English Setter, Tess, sets me up for my shots.
Good luck to you, wing shooting is yet another dimension with a smoothbore and is what the piece is really all about. Smoothbores the versatile gun,
Regards, Paul

Paul's advice is spot on. Many years ago I built a 28 bore for partridge hunting.
[ I foolishly sold it ! ]
It performed far beyond what one would expect for a smaller bore size.
Your .54 is close enough to the 28 , so I predict you'll have a lot of fun with it.

The 28 bore has the same good pattern that a 12 bore has and is light weight. More modern hunters are experimenting with the 28 bore for grouse hunting. The shells are expensive and sometimes hard to find. I would bring my son up to bird hunting with the .28 rather then the .410.
Paul